FIG. 1a shows a bonded joint between a primary structure 40 and a secondary structure 41. Due to fatigue loading, cracks will propagate and follow a path which cannot be previously determined. Three different scenarios may be identified.                The crack 43b propagates through the secondary structure 41, parallel to the bond line 42 (FIG. 1b). This will cause a global failure of the secondary structure 41, which will stop its collaboration with the primary structure 40. Generally this scenario is not catastrophic. If a fail-safe design philosophy is employed, failure of the secondary structure 41 will not generate global failure of the structure, which remains capable of withstanding the external loads.        The crack 43c propagates toward the external surface of secondary structure 41 and extinguishes (FIG. 1c). This scenario can either cause global failure of secondary structure 41 or partially reduce its capability. In both cases, this will not cause global catastrophic failure of the whole structure, as the primary structure 40 remains pristine.        The crack 43d propagates toward the internal surface of secondary structure 41 and then through the primary structure 40 (FIG. 1d). This scenario is not acceptable since it leads to catastrophic failure of the primary structure 40 and therefore must be avoided.        
The ability to confine the crack within an established perimeter would simplify certification activity, increase the level of confidence, improve the reserve factor and the final global weight, and finally, increase safety.
Various self-healing structures are described in “Bioinspired self-healing of advanced composite structures using hollow glass fibres”, R. S. Trask, G. J. Williams and I. P. Bond, J.R. Soc. Interface (2007) 4, 363-371, (doi:10.1098/rsif.2006.0194). A sixteen-ply glass-fibre laminate is described in which self-healing filaments were introduced at four damage critical ply interfaces. A sixteen-ply carbon-fibre laminate is also described in which healing glass fibre (HGF) was located at two interfaces within the lay-up, wound directly onto uncured carbon-fibre reinforced plastic (CFRP) plies prior to lamination. According to Trask et al, the incorporation of HGF as discrete plies was deemed unsuitable for CFRP laminates as it would effectively produce a hybrid glass-carbon laminate and result in a significant reduction to their outstanding mechanical properties.